1. Field of the Invention
The present invention relates to a thermal image-fixing apparatus which thermally fixes an unfixed image such as a toner image on a recording sheet.
2. Related Art Statement
There is known a thermal image-fixing device which thermally fixes an unfixed toner image on a recording sheet in a printer or copier of an electrophotographic type. The image-fixing device includes a heater roller and a halogen lamp. The heater roller is provided by a hollow cylindrical roller formed of a heat-conductive material such as aluminum or stainless steel. The heater roller is rotatable around the halogen lamp fixed in the hollow space of the roller, so that the heater roller receives, from the halogen lamp, an amount of thermal energy necessary to fix the toner image on the recording sheet. The image-fixing device further includes a presser roller formed of, e.g., a heat-resistant silicone rubber. The presser roller cooperates with the heater roller to provide a nipper for nipping, with a suitable pressing force, the recording sheet to which the toner image has been transferred.
The prior image-fixing device further includes a temperature sensor located adjacent the heater roller, and a control device which adjusts, based on the temperature detected by the sensor, an amount of electricity supplied to the halogen lamp and thereby controls the amount of heat generated by the halogen lamp, so that the surface temperature of the halogen lamp is maintained at suitable values. The image-fixing device further includes a thermal fuse located adjacent the heater roller. In the event that the amount of heat generated from the halogen lamp is not controllable because of the occurrence of some trouble with the image-fixing device, the thermal fuse shuts off the supplying of electricity to the lamp, thereby preventing a fire or other safety hazards.
In the prior image-fixing device, however, the halogen lamp heats (a) other portions of the heater roller than a portion thereof corresponding to the nipper where the heater roller contacts the recording sheet and the toner image thereon; (b) bearings which rotatably support the heater roller; and (c) ambient air around the halogen lamp. Thus, the image-fixing device suffers from the following problems:
First, the bearings and other elements for supporting the heater roller, presser roller, temperature sensor, and thermal fuse must be produced using a heat-resistant material such as a resin or a metal. In particular, the bearings for rotatably supporting the heater roller is required to possess a high resistance to heat. In addition, since the overall image-fixing device is subject to high temperatures because of the heat generated from the halogen lamp, the image-fixing device must entirely be covered with a heat-insulative member for protecting a user from being burnt when the user removes a recording sheet jammed at the image-fixing device or the user's hand accidentally touches the image-fixing device. Otherwise, the prior image-fixing device must be disposed at a place where the user cannot touch the device. Thus, the construction of the image-fixing device is inevitably complicated, and the overall size of the same increases.
Second, various elements associated with the image-fixing device must be spaced away from the device, so that those elements are prevented from being adversely affected by the heat generated from the device. In addition, heat-insulative members and/or electric funs must be provided between the image-fixing device and other devices such as a photosensitive body, developing device, scanner, toner cartridge, etc. of the printer or copier. For this reason, the construction of the printer or copier is complicated, and the overall size of the same increases.
Third, it takes a long time for the heater roller to have an image-fixing temperature at the outer surface thereof as a result of conduction of the heat generated from the halogen lamp to the inner surface of the heater roller after application of electric power to the image-fixing device. Thus, a long waiting or building-up time is needed before the heater roller is entirely heated to the image-fixing temperature after the halogen lamp is turned on. This is inconvenient for users who wish to start the image-fixing device as soon as possible.
The fourth problem is that the heat radiated from the heater roller is only partly used to thermally fix the toner image on the recording sheet. A considerable portion of the radiated heat is not used. Thus, the prior image-fixing device suffers from low energy efficiency.
The fifth problem is that in the case where a preheating of the heater roller to a raised temperature for reducing the waiting or building-up time is carried out after electric power is initially applied to the image-fixing device and before an image-fixing operation is actually started, the image-fixing device uselessly consumes electric power while no image-fixing operation is carried out.
Sixth, the prior image-fixing device is composed of elements each having a small thermal capacity, for improving the energy efficiency and/or reducing the waiting time. Therefore, there is a tendency that portions of the heater roller other than a portion thereof contacting the recording sheet are raised to excessively high temperatures. It may be considered that if the cross-sectional area of the heater roller is decreased for reducing the thermal capacity thereof, the thermal conductivity of the roller as taken in the longitudinal direction thereof is lowered, therefore heat does not easily conduct in the longitudinal direction and tends to remain locally on the roller.